This study will test whether high dose oral clopidogrel will produce immediate potent antithrombotic protection in patients with coronary artery disease (CAD) undergoing coronary artery catherization. We propose to establish the dose of clopiogrel and timing after oral administration that immediately inactivates platelet ADP-receptors ex vivo. Each dose will be evaluated in cohorts of aspirin-treated CAD patients undergoing coronary artery catherization.